Mantle amphibole trace-element and isotopic signatures trace multiple metasomatic episodes in lithospheric mantle,western Victoria,Australia

摘要

Peridotite xenoliths from western Victoria,Australia,contain varying proportions of amphibole ( ± apatite).The xenoliths also exhibit varying degrees of metasomatic enrichment as indicated by the rare-earth element (REE) patterns of clinopyroxene and amphibole analysed in situ by laser-ablation microprobe ICP-MS and by whole-rock trace-element patterns.Trace-element patterns allow the samples to be divided into three groups.Group A is characterised by flat REE patterns for whole-rock,clinopyroxene and amphibole,and small negative or absent high-field-strength element (HFSE) anomalies;Group B features high levels of light-REE enrichment,pronounced negative HFSE anomalies and low Ti/Eu in whole-rock trace-element patterns;and Group C shows positive HFSE anomalies in whole-rock patterns.The different trace-element patterns are consistent with metasomatism by different agents or combinations of agents,which have acted upon the already modally metasomatised subcontinental lithospheric mantle (SCLM) beneath western Victoria.Geochemical signatures suggest that Group A samples have undergone silicate melt metasomatism,Group B samples have undergone metasomatism by a carbonate-rich or carbonatitic fluid and Group C samples may record metasomatism by a fractionated silicate fluid with hydrous component.